When USB is disabled, we get a link error for this driver
because of the added OTG support

drivers/phy/phy-rockchip-inno-usb2.o: In function `rockchip_usb2phy_otg_sm_work':
phy-rockchip-inno-usb2.c:(.text.rockchip_usb2phy_otg_sm_work+0x1f4): undefined reference to `usb_otg_state_string'
drivers/phy/phy-rockchip-inno-usb2.o: In function `rockchip_usb2phy_probe':
phy-rockchip-inno-usb2.c:(.text.rockchip_usb2phy_probe+0x2c8): undefined reference to `of_usb_get_dr_mode_by_phy'

Other phy drivers select USB_COMMON for this, so let's do the same
here.

Fixes: 0c42fe48fd23 ("phy: rockchip-inno-usb2: support otg-port for rk3399")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>